The present invention relates to a thermal analysis apparatus used to measure how a physical or chemical property of a sample varies with temperature. More particularly, the invention relates to a differential scanning calorimeter to measure and analyze heat flow excessively generated or absorbed by a sample as compared with a reference when the temperature is varied at a constant rate.
The differential scanning calorimeter is an apparatus which, when a sample and a reference substance (thermally stable substance, usually alumina or the like is used) are symmetrically placed and the temperatures of both are varied at a constant rate, detects and analyzes differentially a heat flow excessively generated or absorbed by the sample as compared with the reference substance.
When the temperature of a sample is raised at a constant rate, heat absorption by the sample increases with an increase in heat capacity of the sample. That is, a differential heat flow signal increases in absolute value. At this time, the absolute value of the differential heat flow signal is proportional to the heat capacity difference between the sample and the reference and the temperature increase rate. Consequently, the heat capacity of the sample can be determined from the differential heat flow signal based on the known temperature increase rate and reference heat capacity.
Meanwhile, when the sample fuses, the heat absorption by the sample temporarily increases. If making a graph on differential heat flow signals recorded in time series, the differential heat flow signals depict an endothermic peak. Also, if following a similar recording method, when crystallization is caused in the sample, the differential heat flow signals depict an exothermic peak. The area of such endothermic or exothermic peak, which is depicted against a time axis set to have a unit time corresponding to a constant length, is proportional to a heat amount released or absorbed (transition heat) upon transition of the sample. Accordingly, if the differential heat flow signal value is calibrated by measuring a known transition heat, it is easy to determine a transition heat of the sample from the differential heat flow signal.
In order to obtain a differential heat flow signal with useful properties as above, the differential scanning calorimeter is broadly used for analyzing a variety of materials. The conventional differential scanning calorimeter is roughly divided into the following two types
One is called a power compensation type. It is structured by a combination of two symmetrically formed independent calorimeters for a sample and a reference, and both are provided with a resistance temperature sensor and heat flow feedback heater. The average value of temperatures detected by the both temperature sensors is compared with a temperature output of a temperature programmer which varies at a constant rate. Two calorimeters are heated up such that the both are brought into coincidence by the heat flow feedback heaters. Also, if a difference is caused in temperature output of the both temperature sensors, the both heaters are immediately increases or decreased in power to return the difference to zero. Thereupon, the difference of power supplied to the both heaters every second is recorded as a differential heat flow signal.
The other is called a heat flux type. It has a heat sink formed of a thermally good conductor within which sample and reference temperature sensors are fixed to form heat paths that are symmetric with and equivalent to each other. The heat sink temperature is compared with the temperature output of a temperature programmer varying at a constant rate, and feedback-controlled by a heater wound around the heat sink such that both are brought into coincidence. The temperature difference between the sample and the reference is detected by a differential thermocouple. On this occasion, if the temperature difference between the sample and the reference is divided by a thermal resistance between the heat sink and the sample, it is possible to determine a differential heat flow as a difference in heat flow to the sample and the reference in a similar procedure to a determination of current by dividing potential difference by resistance. That is, in the heat flux type differential scanning calorimeter, amplification is appropriately made on an output of the differential thermocouple representative of a temperature difference between the sample and the reference to output and record as differential heat flow signals.
The power compensation type differential scanning calorimeter is excellent in responsiveness and can realize a heat compensation time constant of less than two seconds. However, as for the baseline performance, there has been a difficulty in obtaining the same stability as in the heat flux type differential scanning calorimeter. The main reason of this lies in that the power compensation type sensor has a large temperature difference from surrounding members during measurement with a result that a comparatively large amount of heat leak occurs uninterruptedly from the sensor to the outside, causing a drift factor in the baseline. On the other hand, the heat flux type differential scanning calorimeter is excellent in baseline stability, but has a heat compensation time constant exceeding three seconds. Accordingly, there have been the disadvantages that the heat flow signal is blunted at its peak, a plurality of peaks are worsened in separation, and so on.
In order to solve the above problem, the present inventions comprises: a heat sink formed of a thermally good conductor shaving a space for accommodating a sample at an inside thereof: a detector fixed within the heat sink and formed by an insulating substrate formed with symmetric circuit patterns of metal resistors; a temperature measuring circuit for measuring a temperature of the detector by detecting resistance values of the metal resistors in the detector; a differential temperature detecting circuit for comparing resistance values of one pair of metal resistance circuits to detect a temperature difference between a sample and a reference placed in the detector; a program temperature function generator for outputting temperature target values in time; a heat sink temperature controller for controlling a temperature of the heat sink depending on an output of the program temperature function generator; a detector temperature controller for controlling a temperature of the detector by controlling a current value flowing through the metal resistance circuit in the detector based on a comparison result of an output of the program temperature generator and an output of the temperature measuring circuit; and a differential heat compensating circuit for causing a proper current to flow through each of the one pair of metal resistors in the detector such that an output of the differential temperature detecting circuit uninterruptedly returns to zero.
The sample and the reference are roughly controlled by heat conduction through a detector from the heat sink that is controlled in temperature depending on the program temperature. Also, the temperature of the reference is precisely controlled to be brought into coincidence with the program temperature by the detector temperature controller. Furthermore, if a temperature difference occurs between the sample and the reference, the supply power to the heaters separately provided close to the sample and reference is adjusted by the differential heat compensating circuit such that the temperature difference is immediately returned to zero.
As a result of this, both the sample and reference are controlled in temperature according to the program temperature.
The difference in heat generation or absorption of the sample in comparison with the reference is detected as a difference of supply power to the heaters separately provided close to the sample and reference, thus effecting a function as a differential scanning calorimeter.